Which USGI companies used green followers in their mags before the ban went into effect?I know Sanchez did but who else did? I know that post-ban mags had them in them but which pre-ban USGI mags had them already installed?

Sanchez began using GFs in 1989, to see if the new design would fix the feed issues. They did, and GFs were made the standard in late 1991. All USGI-contract mags made after '91 had GFs. This would include:
Okay (who also made Colt mags)
Center
Sanchez
La Belle (military mags, NOT the teflon mags)
FN
-Troy